1. Field of the Invention
The present invention relates to a method and system for utilizing a power transmission network as a communication network along with its normal power transmission function, and, particularly to such a network in which the magnetic field component of the electromagnetic radiation traveling along the power transmission line through a power distribution network is utilized to transmit information between subscribers, including telephonic voice communication.
2.Background Art
The use of power transmission lines as a communication media is well known in the art with some of the earliest work dating back to the 1930's in an effort to provide telephone communication to rural areas which had not yet been served by telephone lines, such as the type of prior art systems described in U.S. Pat. Nos. 2,032,360; 2,264,395; and 2,264,396. These systems, which employed carrier waves superposed on the electrical component of the electromagnetic radiation carried over the power transmission line, were strictly analog in nature and were not particularly successful because of the inherent limitations of the analog bandwidth, signal attenuation, and the inability to handle more than one call at a time. As further attempts were made to develop an analog based system, it was discovered that unsatisfactory radiation from the power transmission line would result at carrier frequencies in excess of 300 KHz which would interfere with long wave radio signals, limiting such systems to a frequency range of 30 KHz to 300 KHz. One such attempt at developing an analog based system, described in U.S. Pat. No. 5,684,450, permitted the use of carrier frequencies greater than 1 MHz for transmission between power transformers, however, it was found that electromagnetic noise from the power lines interfered with the information signals being transmitted, such as each time an electrical appliance connected to the power distribution network was switched on. The need to transmit information through the power transformers also proved an impediment to a successful power line communication system. Attempts have also been made to overcome these problems through the use of digital signal transmission instead of an analog carrier wave, but still utilizing the electrical component of the electromagnetic radiation carried over the power transmission line, such as disclosed in U.S. Pat. Nos. 5,554,968; 5,559,377; and 4,471,399, as well as the use of digital packets to try to overcome the noise problem, but these attempts have also been unsatisfactory and/or too costly.
Because of the well established power transmission infrastructure, and the increasing need for higher speed, higher capacity, and greater distribution of information of all types over great distances, particularly telephonic communications today, there is a continuing and increasing need to develop a satisfactory and cost effective communication system which takes advantage of the vast and well established power transmission line distribution network already in place throughout the United States and the world at large. The disadvantages of the prior art are overcome by the present invention which utilizes the magnetic field component of the electromagnetic radiation carried over the power transmission lines rather than the electric field component which the prior art has concentrated on.